1. Technical Field
The present invention relates to a method for treatment of a gas-flow. The invention is particularly intended for use in the field of purification of exhaust gases in connection with a combustion engine which in particular is adapted for reduction of undesired nitric oxide compounds (NOx compounds) in the exhaust gases.
2. Background of the Invention
In the field of vehicles which are operated by combustion engines, there is a general demand for low emissions of harmful substances in the exhaust gases from the engine. Such substances are primarily constituted by pollutants in the form of nitric oxide compounds (NOx), hydrocarbon compounds (HC), and carbon monoxide (CO). Regarding today's petrol engines, the exhaust gases are normally purified by means of an exhaust catalyst which forms part of the exhaust system and through which the exhaust gases are guided. In what is commonly referred to as a three-way catalyst, a major part of the above-mentioned harmful compounds is eliminated by means of known catalytic reactions. In order to optimize the function of the catalyst so that it provides an optimal degree of purification for NOx, HC, and CO, the engine is in most operating cases operated with a stoichiometric air/fuel mixture; that is, a mixture where *1.
Furthermore, in the field of vehicles, there is a general demand for reducing the fuel consumption of the engine to the highest possible degree. To this end, during the last few years, new types of engines have been developed which are adapted to be able to be operated by increasingly lean fuel mixtures, i.e. where *1. In a direct-injected Otto cycle engine, or DI engine, the combustion chambers in the engine are constructed in such manner that the supplied fuel can be concentrated to a high degree at the respective ignition plug. This mode of operation is generally termed “stratified” operation and during continuous driving at a low or a medium-high torque and engine speed of the engine, it provides an operation with a very lean air/fuel mixture, more precisely up to approximately *== In this manner, a substantial savings in fuel consumption is obtained in this type of engine. The engine can also be operated in an additional, “homogeneous” mode of operation, with an essentially stoichiometric mixture (*=1) or a comparatively rich mixture (*<1). This later mode of operation normally prevails during driving situations with comparatively high torques and engine speeds of the engine.
During stratified operation, a lean exhaust gas mixture will flow through the three-way catalyst. The typical three-way catalyst is not optimal for reducing the NOx compounds in these exhaust gases due to the fact that the three-way catalyst does not function well for purification of NOx compounds during conditions which are rich in oxygen. For this reason, a conventional three-way catalyst can be combined with a nitric oxide adsorber (also called NOx adsorbent, or “NOx trap”). In this manner, the NOx adsorbent can be utilized as a complement to a conventional three-way catalyst, either as a separate unit upstream of the three-way catalyst or as an integral part of the three-way catalyst; that is, together with the catalytic material of the three-way catalyst. In the latter case, an integrated component in the form of a NOx adsorbing exhaust catalyst is formed.
The NOx adsorbent is constructed in such a manner that it takes up or adsorbs NOx compounds in the exhaust gases if the engine is operated with a lean air/fuel mixture and discharges (desorbs) the NOx compounds if the engine is operated with a rich air/fuel mixture during a certain time period. Furthermore, the NOx adsorbent has the property of being able only to adsorb NOx compounds up to a certain limit. It is eventually “filled” and thus reaches a limit for the adsorption. In this situation, the NOx adsorbent must be regenerated; that is, it must be influenced to desorb or release the accumulated NOx compounds. If a conventional three-way catalyst in this case is provided in connection with a NOx adsorbent, or if alternatively a three-way catalyst is formed as an integral part of a NOx adsorbent, the desorbed NOx compounds can be eliminated by means of the three-way catalyst, provided that the latter has reached its ignition temperature. In principle, the conventional three-way catalyst can be arranged either before the NOx adsorbent, after the NOx adsorbent or as an integral part of the NOx adsorbent.
It is known that a NOx adsorbent can be regenerated by means of the exhaust gas mixture which flows through the NOx adsorbent being made comparatively rich for a certain time period, usually a few seconds. This can in turn be achieved by operating the engine with a comparatively rich air/fuel mixture for the time period. In practice, this is achieved by operating the engine during this time period in the homogeneous mode in which the engine is operated on a comparatively rich air/fuel mixture. In this manner, the NOx adsorbent is “emptied” so that it subsequently can adsorb NOx compounds during a certain time period which lasts until a new regeneration becomes necessary.
Typically, a control unit is utilized which functions in accordance with a suitable strategy for switching the combustion engine between homogeneous and stratified operation depending on the degree of throttle application and the speed of the engine, and with regard to whether a NOx regeneration is necessary.
During purification of the exhaust gases from, for example, a DI engine, there is a demand for the capability of controlling the temperature of the NOx adsorbent in order to, among other things, achieve maximal reduction of NOx compounds in the exhaust gases. This is due to the fact that a NOx adsorbent only functions optimally within a certain temperature interval, which in turn depends on the prevailing operating condition of the vehicle. As an example, it can be mentioned that stratified mode of operation in a DI engine, that is, operation with a lean air/fuel mixture, requires that the temperature of the exhaust gases which are guided through the NOx adsorbent lies within the interval of approximately 250-450 degrees C in order for it to be able to operate satisfactorily. A particularly efficient NOx reduction is obtained if the temperature lies within the interval of approximately 300-350 degrees C. Furthermore, there is a general demand for the exhaust temperature not to exceed approximately 800C, which is due to the fact that there is a risk of the NOx adsorbent being destroyed during temperatures which exceed this limit.
The demand for the capability of controlling the temperature generally prevails in connection with other types of engines such as diesel engines and conventional port-injected Otto cycle engines where a correct adjustment of the temperature to the function of the NOx adsorbent is desirable.
One particular phenomenon which arises in connection with a NOx adsorbent is that sulphur compounds (e.g. sulphur dioxide, SO2), which are present in the exhaust gases which are guided through the NOx adsorbent, cause a coating on the active material of the NOx adsorbent. This coating in turn deactivates the NOx adsorbent's capacity to adsorb NOx compounds, which is due to the fact that sulphur compounds are adsorbed instead of NOx compounds. The sulphur compounds originate from the fuel of the engine, and may vary, among other things, depending on the prevailing fuel quality. As a consequence of such a sulphur coating, the adsorption capacity of the NOx adsorbent will be gradually reduced over the course of time.
In order to solve the problem regarding such a sulphur coating, the NOx adsorbent must be regenerated regularly as regards sulphur compounds as well; that is, it must be “emptied” of sulphur compounds causing the sulphur coating on the NOx adsorbent to be removed. In this case, unlike the case regarding the NOx regeneration, it is not sufficient to generate rich exhaust gases in order to achieve this sulphur regeneration. Instead, a sulphur regeneration can be accomplished by operating the engine during a certain time period so that it generates a rich exhaust gas mixture (i.e. *<1) at the same time as a comparatively high exhaust gas temperature is generated. More precisely, during an exhaust gas temperature that is higher than approximately 650 degrees C, and preferably within the interval of 650-750 degrees C. In this manner, sulphur compounds can be desorbed or discharged from the NOx adsorbent so that it can once again be utilized with a satisfactory adsorption capacity for NOx compounds.
Traditionally, the sulphur regeneration is preferably made with a suitable time interval which is determined on the basis of the lost NOx storage capacity of the NOx adsorbent, which in turn can be estimated on the basis of the sulphur content of the fuel being used in the vehicle and the vehicle's fuel consumption.
Thus, there is a problem in connection with known engine systems attributable to the difficulty in combining the required exhaust gas temperature during lean driving (approximately 250-450 degrees C) with the demand for a suitable temperature for sulphur regeneration (approximately 650-750 degrees C), while at the same time controlling the temperature so that it does not exceed the higher limit value of approximately 800 degrees C.